At present many types of pyrotechnic charges are found, hereinafter also referred to as primers, which are used in civil and/or military applications. Civil applications can be, for example, gas generators for airbags or safety belt stretchers, in which a pyrotechnic charge is used to generate gas. Military applications can be artillery shells in which a pyrotechnic primer forms part of an ignition chain for detonating an initiator.
A pyrotechnic charge comprises a mixture of at least one reducing agent, hereinafter also referred to as fuel, and at least one oxidizing agent, hereinafter referred to as oxidizer, as well as further additives such as, for example, bonding agents and burning rate moderators. In conventional pyrotechnic charges, the fuel and the oxidizer are normally comprised as a powder mixture. More recently, however, new types of pyrotechnic charges have been developed, in which the fuel consists of a coherent porous fuel structure and in which the oxidizer fills the porous structure. The material in the porous fuel structure is constituted, for example, by silicon, and the porosity lies within the range 50-70% by volume, in certain cases the porosity exceeding 70% by volume (highly porous structures).
In patent specification US 2003/0148569 A1, a pyrotechnic charge is described, which pyrotechnic charge is produced by a saturated solution of lithium nitrate being fed to a coherent porous silicon structure. After the solution has been absorbed in the porous fuel structure, the solvent is distilled off and the oxidizer is precipitated as fine crystals in the porous silicon structure.
The advantage of utilizing a coherent highly porous fuel structure is that a large specific contact surface area between the fuel and the oxidizer is achieved. The large contact surface area means a high availability of the oxygen of the oxidizer during the combustion, which means a low slag component in the combustion products and little effect upon the environment.
A drawback is, however, that the large specific contact surface area between the oxidizer and the highly porous fuel structure strongly increases the sensitivity of the pyrotechnic charge to impacts, shocks and static electricity, which in turn increases the risk of unintentional ignition.
The increased risk of unintentional ignition can have serious consequences if a pyrotechnic charge of the said type is used, for example, in an artillery shell or in an airbag system for vehicles.
The technical problem on which the present invention is founded has been the risk of unintentional ignition of pyrotechnic primers comprising at least one coherent porous fuel structure and at least one oxidizer.